Polyethylene terephthalate (PET) films exhibit excellent properties for use as photographic film base with regard to transparency, dimensional stability, mechanical strength and resistance to thermal deformation. However, PET films are extremely tough and not well suited for finishing operations, i.e., slitting, chopping and perforating processes, which are required in the preparation of photographic films.
The process for making a polyethylene terephthalate film typically comprises the steps of casting a molten polyethylene terephthalate resin in a machine direction onto a casting surface to form a continuous sheet, drafting the sheet by stretching in the machine direction, tentering the sheet by stretching in the transverse direction, heat-setting the drafted and tentered sheet, and cooling the heat-set sheet to form a stretched, heat-set polyethylene terephthalate film, such as described in, e.g., U.S. Pat. No. 4,141,735, the disclosure of which is incorporated by reference herein. U.S. Pat. Nos. 5,385,704 and 5,607,826 disclose a method for improving the finishing characteristics of photographic materials employing a PET film base having reduced fracture resistance involving lowering the planar birefringence of the film base to below 0.150 by performing a detentering step which allows the tentered film to shrink in width by 2 to 20% (pref. 10-18%) after the heat-setting step during film manufacturing. Detentering, however, cannot be applied in some circumstances, e.g., when the width of the web must exceed some minimum value or when the film production machine is not properly equipped for conducting such an operation. Copending, commonly assigned U.S. Ser. No. 09/223,876, filed Dec. 31, 1998, discloses a method for making PET film base employing relatively high heat set temperatures which results in reduced fracture resistance without the need for substantial (e.g., more than 2%) detentering, and demonstrates improved perforating performance for such supports with respect to the generation of dirt. Preferred drafting stretch ratios of from 3.0 to 3.5 and tentering stretch ratios of from 2.8 to 3.3 are specified in the above referenced U.S. Pat. Nos. 5,385,704 and 5,607,826 and U.S. Ser. No. 09/223,876.
The photographic industry has long recognized the need to provide photographic elements with some form of antihalation protection. Halation has been a persistent problem with photographic films comprising one or more photosensitive silver halide emulsion layers coated on a transparent support. The emulsion layer diffusely transmits light, which then reflects back into the emulsion layer from the support surface. The silver halide emulsion is thereby reexposed at locations different from the original light path through the emulsion, resulting in "halos" on the film surrounding images of bright objects.
One method proposed for antihalation protection in photographic films comprises providing a dyed or pigmented layer behind a clear support as an antihalation backing layer, wherein the backing layer is designed to be removed during processing of the film. Typical examples of such antihalation backing layers comprise a light absorbing dye or pigment (such as carbon black) dispersed in an alkali-soluble polymeric binder (such as cellulose acetate hexahydrophthalate) that renders the layer removable by an alkaline photographic processing solution. Such carbon containing "rem-jet" backing layers have been commonly used for antihalation protection in motion picture films. The carbon particles additionally provide antistatic protection prior to being removed. While such rem-jet backing layers provide effective antihalation and antistatic protection for photographic films prior to processing, their use requires special additional processing steps for their subsequent removal, and incomplete removal of the carbon particles can cause image defects in the resulting print film. Additionally, it is often desirable to provide "process surviving" antistatic protection for photographic elements in order to prevent static build-up even after imagewise exposure and processing, especially for motion picture films which are subject to rapid transport through projection apparatus where static charges can attract dust particles which may detrimentally impact a projected image.
Accordingly, alternatives for carbon-containing, process-removable, antihalation/antistatic backing layers have been proposed. One such alternative is to use antihalation undercoat layers containing filter dyes coated between the support and the emulsion layers wherein the filter dyes are solubilized and removed and/or decolorized during processing of the film, and a separate process-surviving antistatic backing layer, such as described in U.S. Pat. Nos. 5,679,505 and 5,723,272. Dyes may be selected and used in combinations to provide antihalation protection throughout the visible spectrum. Process-surviving antistatic layers typically include, e.g., ionic polymers, electronic conducting non-ionic polymers, and metal halides or metal oxides in polymeric binders. Conductive fine particles of crystalline metal oxides dispersed with a polymeric binder have been found to be especially desirable for preparing optically transparent, humidity insensitive, antistatic layers for various imaging applications.
Photographic elements comprising PET film supports, an antihalation undercoat, and a process-surviving antistatic backcoat have been found to be particularly sensitive to finishing operations. It would be desirable to provide a PET film base for use in a photographic element with antihalation undercoat and process-surviving antistatic backcoat layers which provides improved finishing characteristics, especially a film base which provides for photographic elements with an improved overall dirt position after both slitting and perforating operations.